Flood Risk Above Average in Red River of the North, Northern Rockies, Northwestern Colorado, Ohio Valley and Northern Maine

(As of March 10, 2006)

Saturated soils, substantial water in the snowpack (well above average), and streamflows well above normal have produced a risk of moderate to major flooding along the Red River of the North. The flood risk for this spring is above average across  the the Red River of the North in North Dakota and Minnesota, Great Basin and parts of the Rockies, northwestern Colorado,  parts of the Ohio Valley, the Pearl River in Mississippi and parts of northern Maine. Substantial snowpack and water held in the snow across northern portions of the West are the factors contributing to the above average risk. The potential for ice jams has increased in northern New England. In Maine, 1 to 2 feet of river ice present an ice jam flooding risk. Recent very heavy rains across the Ohio Valley region have produced flooding, with little additional precipitation needed to exacerbate the current flooding. The flood risk for much of the remainder of the nation is below average with some areas of average flooding risk. 

Ongoing drought conditions have contributed to the below average flood risk across the southwestern part of the nation. Recent dryness has caused the reduced flood risk across much of the Mid Atlantic. For the central portion of the country, an average flood risk is present in the upper and middle Mississippi River drainage area. The remainder of this region has a below average flood risk. Flood risk is gauged by analyzing hydrometeorological conditions such as precipitation, groundwater conditions, streamflow, snow conditions, river ice, and reservoir storage.

Rainfall across the U.S. has varied from less than 25% of normal during the past 90 days in parts of the Southwest to 175% of normal over parts of the Northwest quadrant of the U.S. More recently, precipitation totals have been sub-normal over much of the nation. However, a series of storms this week have dropped heavy rain across the Ohio Valley, causing rivers to flood.  Along the Pearl River in Mississippi/Louisiana, the current flooding contributes to the above average flood risk, in an area where there is ongoing recovery from hurricane damage. 

Temperatures have been exceptionally warm this winter, especially across the north-central region. This warmth has melted much of the snow which has fallen this winter so far, with the only notable accumulations in the higher elevations of the Northwest, the northern Plains and Great Lakes, and northern Maine. Cooler conditions in the past month across much of the nation increased frost depths in the northern Plains but which remain substantially less than average. Winter returned to the Northeast during late February, increasing ice and snow depths. Storms in mid February across the north central U.S. have dropped up to 18 inches of snow. However, recent temperatures have again caused substantial melting of this new snow. Both the snow depth and the amount of water held in the snow is markedly below normal for this time of year in much of the Northeast, Mid-Atlantic and north central part of the U.S. In contrast, the Red River of the North basin has above average amounts of water held in the snow as do portions of northern and western Maine and western New York. Based on the current snowpack there is an above average risk for flooding across parts of the Great Basin and northern Rockies. 

Plentiful rainfall has resulted in very wet soils in the Northeast, parts of North Dakota, Minnesota and Michigan as well as much of the northwestern quadrant of the country. This wetness is reflected in the Palmer Drought Index, as well as soil moisture conditions used by the National Centers for Environmental Prediction in their forecast models. These conditions have produced nearly saturated soils, and any additional rain can run off quickly and result in flooding. In particular, the headwaters of the Red River of the North saw more than three times normal precipitation last autumn, leading to exceptionally wet soils and enhancing flooding risks. Below average precipitation from parts of the Southwest across the southern and central Plains into the South and Mid-Atlantic has led to drying to very dry soils, and reduced flood risk in these areas. 

Rivers across New England and the northern Mid-Atlantic are running normal to above normal while southern Mid-Atlantic are generally running below normal levels for this time of year. Flows in the Red River of the North Basin are running well above normal levels, with above average flows also in parts of the coastal West, due to recent heavy rains. The warmer than normal weather through the winter has caused river ice to be less extensive and not as thick as normal across the northern states. The cold temperatures in February (although still above normal) did not enhance the river ice significantly in the Northeast. The recent cold temperatures have enhanced ice thicknesses in Maine, New Hampshire, Vermont and portions of New York and filled in some open stretches. The significant ice is found in northern Maine, where some 1 to 2 foot thicknesses were measured. However, river ice thicknesses are below normal for this time of year. Further to the south and west, very little river ice is observed. In the central U.S. river ice on the Red River is ranging from 9 to 18 inches, well below average, while colder temperatures in February led to a thickening of this ice. This ice is currently breaking up in the southern basin due to reservoir releases. Elsewhere, little or no ice is being reported on area rivers.

Water supply across the West is varied this year. Substantial precipitation across the northwest quadrant of the nation has produced average to above average snowpack to date, while the Desert Southwest is nearly void of any snow. Reservoir storage as of March 1st compared to the March, 2005 storage shows some recovery from the multi-year drought across the west. The plentiful precipitation last year across Arizona and Nevada produced the above average reservoir storage, despite this year's massively sub-normal snowpack.

As a result of substantial rainfall during the fall and winter, reservoirs across the Northeast are running at or above the usual winter levels. The New York City water supply system is at 95.1 percent of capacity, about 9 percent above the long term normal level for this time of year. Five of the seven major reservoirs comprising the New York system are above capacity and are spilling into the drainage basins below. Other reservoirs across the Northeast report levels well above average if not at capacity. The Quabbin Reservoir in southern New England, the largest water supply reservoir in New England was overtopping the spillway as of 2 March. Most large water supply reservoirs within the mid Atlantic are holding average or above average storages for this time of year, while most flood control reservoirs are generally close to normal levels for late winter. Reservoirs across the north central U.S. are performing their winter draw down to allow capacity for the spring runoff.

Across Alaska, the risk of flooding due to snowmelt or ice jams is currently rated as average.  The snowpack is below normal to normal across mainland Alaska, with ice thicknesses near normal.

In Summary; the recent heavy rain has caused ongoing flooding in the Ohio Valley and extending into parts of Illinois, Indiana and Kentucky, with an elevated risk for additional flooding this spring. Existing river ice with substantial water held in the snow pack and wet antecedent conditions leads to an above average risk of flooding along the Red River of the North in North Dakota and Minnesota. Plentiful snowpack in parts of the West has led to average to above average flood risk, while areas with sub-normal amounts expect little flood risk. The combination of recent heavy rain and saturated soil conditions compensate for the lack of snow leading to an average to below average flood risk in parts of New England and the Mid-Atlantic, with only parts of northern Maine at elevated risk for flooding due primarily to river ice and snow cover. A below average risk exists from western New York southward through the Mid Atlantic where the warmest January in 56 years combined with the continuing warmth has left the ground bare of snow and melted river ice, the two main causes of flooding in the region. The lack of any significant snow or river ice combined with below normal precipitation across the Mid-Atlantic and much of the central U.S. has resulted in drying soils and lower stream flows, reducing the flooding risk in this region.

It is important to remember that heavy rainfall at any time can lead to river flooding, even when overall river flood potential is considered below average and ice jams can cause rapid water level rises on rivers with significant ice accumulation.